Ethylene polymers such as homopolyethylene and linear low density polyethylene (LLDPE) are excellent in a transparency, a mechanical strength and the like and widely used as a film and the like. Various production methods for the ethylene polymers have so far been known, and it is known that use of a solid titanium catalyst comprising a titanium catalyst component containing titanium, magnesium, a halogen and an electron donor as an optional component makes it possible to produce ethylene polymers at a high polymerization activity. In particular, it is described in a patent document 1 that a catalyst for ethylene polymerization comprising as a titanium catalyst component, a solid titanium catalyst component obtained by bringing a halogen-containing magnesium compound prepared in a liquid state into contact with a liquid titanium compound and an organic silicon compound having no active hydrogens shows a high activity. Further, it is described in a patent document 2 that polymers having excellent particle properties are produced by using an olefin polymerization catalyst comprising an aluminum compound selected from aluminosiloxane, a reaction product of alkylaluminum and calixarene and a reaction product of alkylaluminum and cyclodextrin, a halogen-containing magnesium compound and a titanium compound.
On the other hand, if ethylene can be polymerized at a higher activity in producing the ethylene polymers, not only the productivity is improved, but also a catalyst residue per polymer, particularly a halogen amount is reduced, and therefore problems such as generation of rust on a die in molding can be resolved. Accordingly, a titanium catalyst component which can polymerize ethylene at a higher activity is desired.
Polymers obtained by polymerizing ethylene are usually obtained in a powder form regardless of a slurry method, a gas phase method and the like. In this case, they are preferably ethylene polymers which do not contain fine powders and have a narrow particle size distribution and which are excellent in a particle flowability. The ethylene polymers which are excellent in particle properties have various advantages that they can be used as they are without pelletizing depending on uses.
Further, known is a method in which a molecular weight distribution is broadened by multistage polymerization in order to obtain a film which is excellent in a transparency and a mechanical strength. Usually, a molecular weight is controlled by adding hydrogen, but the activity tends to be reduced when elevating an amount of hydrogen to produce a low molecular weight part. That is, a catalyst which can control a molecular weight with a small amount of hydrogen is advantageous in terms of an activity even in multistage polymerization. Accordingly, an ethylene polymerization catalyst having an excellent property of controlling a molecular weight by hydrogen, which is called a hydrogen response, is desired.
Also, a polymerization solvent-soluble component tends to be increased when producing a low molecular weight part by multistage polymerization, and by product comprising the soluble component is desired to be reduced for the sake of a yield of the product and environmental correspondence.    Patent document 1: JP 1997-328514A    Patent document 2: JP 1998-53612A